About Me

No, not THAT Bob Hoover :-) (ie, Robert A. "Bob" Hoover from Tennessee and perhaps the best pilot in the history of flight.)
The problem is that all Roberts get Bob-ed at birth and there isn't much we can do about it. When posting something about aviation I generally use 'R.S.Hoover' to prevent confusion.

Monday, July 23, 2007

If you’ve never installed nor removed piston rings, buy yourself a piston ring tool like the one in the photo. Harbor Freight sells them. (The tool in the picture is about forty years old. It still looks new because I generally use my hands.)

Small pistons, you can use your bare hands to install/remove the rings... if you know how and are only doing one engine at a time. If you do a lot of engines you generally use a tool. Some piston rings have over-lapping ends, others are tapered and you may be forced to use a tool since your thumbnails can’t get a grip on the oddly shaped gap. When the oil scrapers are steel, as they are on this particular set of jugs, you use your hands regardless. Steel oil-scraper rings are very flexible and must come off before the corrugated band can be removed (and are installed after it is in place). (The compression rings are cast iron and are quite brittle. Using the tool can save you some grief.)

Provide yourself with pencil, paper and four baggies; sandwich bags will work fine. Then round up the most accurate scale you can find. You’ll also need a coffee can filled with enough mineral spirits to submerge a piston, some means of removing metal from the piston and about half a sheet of #600 wet & dry sandpaper. If you have a handy method of holding the piston you can use a rotary file for the coarse removal but you’ll need flapper wheels or some other abrasive removal method for the fine work. You’re also going to need a small, fine-grained whetstone; the sort of thing you’d use to sharpen a pen knife. A 3x loupe or reading glass will come in handy. Finally, you need a set feeler gauges. But just round up the last three; we probably won’t get to use them during this session.

(Note: They rings must fit their groove, in that you don't want a 1mm ring in a 2mm grove. They should rotate easily. If they don't, try soaking them in solvent. Often times cosmoline or other preservative will harden between the ring the land, locking the ring in place. Ring/groove clearance varies from slightly over a thou to as much as four thou, depending on the application. For a low rpm engine such as the one described here, so long as the ring rotates freely in its groove, it should work okay.

While that may sound a bit casual to some, optimum ring clearance is usually obtained by lapping over-size rings for a precise clearance to forged racing pistons. While the time and expense may be justified for a racing engine such close tolerance in this particular area can be a detriment to the durability of a low-rpm engine.)

Make yourself four small paper tags about the size of a double-wide postage stamp. Number the tags 1 through 4. Put one in each of your bags. Start with your #1 piston and remove the top ring. That is, find the ring-gap, push the other side of the ring fully into its groove so that the gap is exposed on this side and grab a’hold of it with your piston ring tool. Gentle squeeze whilst pushing toward the piston to clear the ring from its groove, then lift it straight up and off the piston. Relax your hand and there’s the compression ring laying in the tool.

DON’T MOVE IT. We want to keep it right-side up

Some rings are marked ‘TOP’ to give you the hint. (This set was.) Others have a dot or something to indicate this side up. And some don’t have a damn thing to go by. Which is why I don’t want you to alter the orientation of the ring.

You know you’re looking at the top of the thing because you’ve just removed it. Right now we don’t know if it was installed correctly or not but we’ll give them the benefit of the doubt. Inspect the ring near the gap. Ideally, there will be some kind of marking to indicate the top-side. If yes, then drop on down to the next step. If not, wipe the oil off the ring near the gap and use your pencil to make your own mark.

Now we know Which Way is Up, putting us ahead of at least 50% of the population :-)

NEXT STEP... is to examine the cross-section of the ring. Compression rings can take any shape and often do. Look for a step or angle on the inner edge of the upper ring. If you see something like that I want you to make a sketch of it. Doesn’t have to be artsy-fartsy just a simple drawing showing the shape of the compression ring relative to its orientation. (We already know which way is up, right? So that’s at the top of your drawing, meaning don’t do a Dali on me and drawn the damn thing upside down with a purple horse in the background. ) But keep it neat. Put the date on it, the engine’s serial number if it’s got one, and your name. Your printed name. This becomes part of the engine’s documentation package.

Once you know you can identify the compression ring, put it in the baggy with the #1 sticker and remove the second ring. Go through the same inspection, marking and identification process. Only difference here is that the Second Ring usually has an angle, groove or step on the outer face (and usually on the lower side, but not always.)

The bottom ring is the oil control ring and can take a wild variety of forms. Most recently, Mahle has been using a pair of thin oil-scraper rings separated by a corrugated steel band with color-coded ends. The oil scrapers can usually be installed any which way; most don’t have a preferred up or down so just remove them, then the corrugated band, and bag them.

With just three rings, you’re getting off easy :-) Some pistons have as many as six(!)

Finally, push out the wrist pin and add it to the contents of the baggy. Wrist pins are usually identical in weight to better tan a tenth of a gram.

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Follow the procedure above for the remaining three pistons.

WHAT DOES IT WEIGH?Got your scale? Okay, zero it then weigh each of the piston pins to confirm they all weigh the same. If your set of pins isn't identical in weight they you’ll have to make them so before going on. But 999 times out of 1000 they’re either identical or with 0.1g, which is small enough to ignore. If yours are not then drop me a line and I’ll lead you through the balance procedure.

With a clean pallet and clean piston, determine their weight. Write it on the top of the piston. After doing all four, re-set your zero and weigh them again, this time to confirm the first measurement. If the second weighing differs from the first, figure out what you’re doing wrong, correct it, erase the figures you’ve recorded and start over.

You should get something like this:

#1 = 386.4#2 = 391.0#3 = 389.1#4 = 388.2

Subtract the smallest from the largest: 391.0 - 386.4 = 4.6 grams. A little over a quarter of an ounce; small enough to have the shade-tree types doing hand-stands. But if you’re a serious builder of good engines, it’s at least 4.5 grams too much.

With static mass-balancing the idea is to reduce the weight of the heavier parts to match the weight of the lightest part. That means you only have to balance three pistons, not four.

Start with the heaviest piston and remove metal from the edges of the skirt and from the balancing pads inside the skirt. Use a rotary file or a coarse flapper wheel. When you get to within 1 gram of your goal, switch to a fine flapper wheel. When you get to within a couple of tenths of your goal, wash the piston in white mineral spirits, blow it dry and confirm its weight. Remove the final fractions of a gram by hand using #600 wet & dry sand paper, smoothing the areas where you’ve already removed metal.In the final stages, each time you weigh the piston you must make sure it is clean. The residue of metal you’re removed will try to cling to the piston. It will get into the ring grooves and wrist-pin trunnions. And when it does, its weight will still be there.Balance all three of your heavy pistons. Try to hold to zero, plus or minus 0.1 gram.

After balancing, clean the pistons, re-insert their wrist pins and put them back into their original bags.

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About the third or ninth engine you’ll start thinking there’s gotta be a better way to do this balancing shit. And there is, if you have a small lathe, meaning something with at least a 3" swing. What you do is rig a collar of soft copper to accept whatever size of piston you’re balancing then modify a boring bar to reach up under the piston’s skirt and remove metal from the balancing pads. Chuck the piston in the collar, zero the clock on your carriage (so you can determine your depth by thousandths of an inch), and start cutting. Ruining a few junked pistons will have told you how much weight you are removing for each 1/1000". To balance your jugs all you gotta do is watch the clock.

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All sorts of people are going to tell you that balancing is a waste of time. These are usually the same folks trying to sell you over-priced un-balanced dune-buggy engines; the ones that rust-out before they wear out :-)

Some people want to balance their rods and pistons but don’t have a precision scale. A few have tried the simple balance-beam scale depicted in the ‘Idiot’s Guide’ only to discover there’s a bit more to it. Well... you can build yourself an accurate beam-balance but you have to be a pretty good metrologist to get repeatable results. So instead of shooting for the moon and missing why don’t you try to shooting for a goal you can achieve? Such as using one of those inexpensive gram-scales that are only accurate down to 2.0 grams. Okay, that’s twenty times worse than 0.1 gram but you’ll still end up with a better engine. Here; lemme show you why:

Weighing the same pistons above on a common postal scale having a resolution of two grams (about 1/8 of an ounce), I got the following:

#1 = 388#2 = 392#3 = 390#4 = 388

At the very least you will be able to reduce your imbalance to the resolution of the scale, or 2.0 grams. The resulting engine won’t be perfect but it will be one hell of a lot better than one which hasn't been balanced.

Finally, there’s the guys who are afraid to modify anything. In that case, just weigh the damn pistons and think about it for a while. The VW engine is a ‘boxer’ design - - the crank-throws are paired. When cylinder #4 is at TDC it’s opposed twin, cylinder #2, is also at TDC (although on a different cycle). Clearly then you do not want to pair your heaviest piston opposite your lightest. If you installed work-number-1 in the #4 cylinder then you want to install the next heaviest piston in cylinder #2. That would be the work-number-3 piston (389.0 grams). So without doing anything at all you’ve managed to reduce the maximum imbalance for that PAIR from 4.6 grams to just 3.0 grams. The other pair comes out even better: just 1.8 grams between them. Here again, it isn’t perfect but it is better than trusting to luck.